The present invention relates to a high speed thermal printer for printing by varying the printing speed in accordance with maximum gradation data on a line-by-line basis.
In general, a thermal video printer prints characters or graphics images on a printing paper by sublimating dye contained in a film with energy supplied to a thermal print head, wherein the greater the thermal energy delivered to the paper, the higher the printing density. Such a sublimating thermal video printer is typically capable of printing in 256 gradation levels for each of the primary colors Y (yellow), M (magenta), C (cyan) for each pixel.
Among the conventional thermal video printers, a video printer is disclosed in U.S. Pat. No. 4,990,930 which detects a maximum value from the image data of a line, drives a stepping motor at a constant speed until the maximum value has been printed, and then drives the motor at a predetermined speed to the start position of the next printing line. This apparatus will be explained hereinbelow with reference to FIGS. 1 through 5.
FIGS. 1 and 2 show the relationship between the motor speed (a constant speed) in a normal condition and a driving pulse driving the motor. That is, if 8-bit image data is used to express gradation levels 1 through 256 (256 being the maximum), a motor is driven at a constant speed for the duration of printing the gradation levels of 1 through 256.
Referring to FIGS. 3 and 4, if the detected maximum data value corresponds to gradation level 128, the motor is driven at a constant speed until the gradation level 128 has been expressed, and in an acceleration frown time t.sub.0 representing the finishing of printing one line to time t.sub.1 representing the start of printing the following line, which results in the discontinuously dye-transcribed section as shown in FIG. 5.
That is, printing at the maximum gradation level 256 causes no problem in practice because the dye is uniformly spread throughout the entire printing time. However, for example, printing at the gradation level 128 requires the dye to be expressed only for the duration of the first half of the printing time (corresponding to one-line printing). As a result, linear stripes are generated in the main scanning direction (vertically along the direction of movement of the thermal print head) throughout the image, which degrades the image.
Moreover, this video printer has a further drawback of requiring a long printing time. The printer performs printing at a constant speed until the detected value of maximum gradation has been printed, as a result of which even for accelerated transporting of the printer head to the start of the next printing line after a preceding line of the image has been printed, a certain amount of time is necessary for the printer head's movement.
In addition, a printing time control method for shortening the printing time in video printers is disclosed in the Japanese Laid-Open Patent Publication Hei. 2-112960. In accordance with this method, whether or not the data has been applied to the thermal print head for each gradation is detected, and the printing operation is finished when the gradation data becomes null. However, such a video printer also requires significant time to transport the printer head to the start of the next printing line after printing a preceding line.